Certain types of material handling machines incorporate electric drive systems for moving the machine from location to location and/or for moving a machine "substructure" on the machine itself. An example of such a material handling machine is an overhead travelling crane (OTC) used in factories, steel handling bays or the like for lifting and placing loads. Such a crane traverses along a pair of elevated main rails which are parallel and spaced apart, usually by several yards. A pair of bridge girders extends between the rails and there are driven wheels mounted at either end of the girders for riding atop the rails. And the girders themselves have rails on them.
A substructure called a "trolley" is mounted on the girder rails and traverses the width of the bridge under motive power. A load hoist is mounted on the trolley and includes a powered hoist/lower "rope drum" or drums about which steel cable is spirally wrapped. The cable is connected to a load-lifting hook, sling, bucket, magnet or the like. With the foregoing arrangement, the operator (who usually rides in a cab which is attached to and moves with the bridge) can pick up, move and deposit a load anywhere in the area travelled by the crane. Other, somewhat less common operating options include radio-controlled cranes operable from the ground or other remote location and operator cabs which are trolley, rather than bridge, mounted.
Another, similar type of crane is called a straddle crane and many of its operating principles are similar to those of an OTC. A difference is that the main rails are near ground level and the bridge and trolley are supported at an elevation by legs extending between the bridge girders and the main rails. A straddle crane resembles an inverted letter "U" in shape.
And there are yet other types of OTCs. One is often called a half-straddle or half-gantry crane in that one end of the crane bridge is supported by an elevated rail while the other is supported by a downwardly-extending leg, the steel rail-car-like wheels of which ride atop a parallel rail near ground level.
An exemplary overhead crane employs two electric-motor traverse drive systems, one each for the bridge and trolley traverse drives. A third electric-motor drive system is used for hoisting and lowering loads. Such drive systems may be powered by direct current (DC) or alternating current (AC). While DC drive systems were almost universally used in older steel mills and the like, AC variable frequency drive systems are becoming increasingly common, at least in part because of the advantages of precision control and design flexibility which they offer. In a variable frequency drive system, motor speed is a function of the frequency of the electrical voltage applied to it. Examples of AC variable frequency drive systems (used for hoist drives) are described in U.S. Pat. Nos. 4,965,847 (Jurkowski et al.) and 5,077,508 (Wycoff et al.). The leading manufacturer of overhead cranes and AC drive systems therefor is Harnischfeger Industries, Inc. of Milwaukee, Wis. One such AC drive system is sold under the trademark SMARTORQUE.RTM. and the invention involves a modification of a known type of SMARTORQUE.RTM. controller.
Even with no load on the hoist, bridges and trolleys of overhead cranes are very heavy. And with a crane-suspended load, the total weight can be much greater. Overhead cranes are designed to lift loads ranging from a few tons to well over one hundred tons. And it should be appreciated that unlike the relatively flexible tires of an automobile on a hard-surface road, steel wheels running on steel rails provide very little resistance to rolling, i.e., very little slow-down effect. Clearly, the crane operator must be provided the ability to retard and stop bridge or trolley motion in a controlled manner.
To that end, AC drive systems are often provided with a feature called "regenerative braking." With regenerative braking, the kinetic energy of the moving bridge or trolley is dissipated in a large resistor bank as heat or is "pumped back into" the AC power line. In either instance, the effect is to retard the bridge or trolley traverse drive (and, thus, the bridge or trolley itself) at a rate much more rapid than would occur simply by letting the bridge or trolley coast to a stop. And when a crane is turned off (at the end of a shift, for example), spring-set brakes prevent drive motor rotation. Such brakes are electrically released but also include a controlled-braking capability. That is, the crane operator can apply the brake in a way much like automobile brakes are applied.
Bridge and trolley traverse drives are operated by an electrical controller coupled to an operator-manipulated master switch in the cab. Such master switch has a handle with a neutral position and a series of positions in either of two directions from neutral. The handle thus controls drive speed in either of two directions. And the farther the handle is moved away from the neutral position, the faster the traverse drive moves the bridge or trolley.
Regenerative braking occurs whenever the actual speed of the traverse drive is greater than the speed then "set" by the master switch handle. And in that event, the electric motor and controller "ramp down" or decelerate the speed of the bridge or trolley traverse drive to the set speed. The quoted expression derives its name from the fact that when depicted on a two-axis graph, the line representing rate of deceleration slopes and is therefore ramp-like in shape. Usually, the control manufacture sets such rate--it is not changed in day-to-day crane operations.
Regenerative braking is not the only means available to retard bridge and trolley motion. Usually, an electrohydraulic brake (as described above) is coupled to each of the crane bridge and trolley traverse motors. As with an automobile, the operator can depress a foot pedal in the cab and brake the traverse drive at a rate selected by pedal foot pressure. And sometimes (indeed, very often in certain types of material handling operations) the operator must retard the traverse drive and brake it to a stop in a much shorter time than would result from the use of regenerative braking alone. It might be said that the operator needs a rate of deceleration with a steeper slope.
However, the provision of redundant retarding capabilities (by regenerative and foot-pedal braking) has resulted in some traverse drive operating problems. In the event the crane operator uses the hydraulic foot brake to try to decelerate the bridge or trolley at a rate more rapid than set by the drive controller, the traverse drive and the foot brake "fight" one another.
This is so since the controller (with its pre-set rate of deceleration) will regulate the drive to decelerate "along the ramp," neither faster nor slower. If the hydraulic brake is very lightly applied, the controller will continue deceleration by regenerative braking, aided to a slight degree by the hydraulic brake. On the other hand, if the hydraulic brake is aggressively applied (and thus tries to slow the crane more rapidly than it would normally decelerate "along the ramp"), motor operation shifts from what is known as the regenerative quadrant to the motoring or driving quadrant. That is, the motor again starts driving the crane and "drives through" the hydraulic brake.
To again use the analogy of an automobile, the effect is like letting up on the vehicle accelerator at a relatively slow rate (regenerative braking) but applying the brakes with the other foot at a more aggressive rate. The vehicle engine (even though slowing) tries to drive the vehicle at a more rapid rate than is indicated by brake pedal pressure.
In an overhead crane, the overly-aggressive level of retarding torque which can be imposed by the hydraulic brake can severely stress and damage the electrical controller itself, the drive motor and/or the hydraulic brake. Overhead cranes represent substantial capital investments and their economic use depends in large part upon the ability to keep them working. In other words, "uptime" is critical and crane downtime can mean the difference between profitable and unprofitable operations. And repair components are expensive. A large bridge drive motor may cost several thousand dollars and weigh hundreds of pounds.
An improved method and apparatus for preventing dual braking in such material handling machines would be an important advance in the art.